Ramesh: 'I now live under a tree ... my parents still work because I am of little use now.' Photograph: Anil.

Rapalhi Srinivas hates his father very much. He tells us this immediately after divulging his name. What follows is a harrowing account of a desperate childhood in a state in south India. Rapalhi left home at a young age, after many fights with a father who refused to let him study. Subsequently, he suffered abuse from members of his family, rape at the hands of his uncle, and homelessness, before returning to care for his HIV-infected father. Eventually, he received his own life-shattering HIV diagnosis. At the end of this grim biography, Rapalhi states simply that he wants 'to make sure that people's lives don't end up like mine'. Rapalhi has taken part in Unheard Voices, Hidden Lives, an extraordinary participatory photography project involving people living with HIV in some of the countries most affected by the epidemic: Cambodia, Ecuador and India.

Nearly 40 million people worldwide are living with HIV; 250,000 of them are from Cambodia - the country most affected by HIV in Asia, with a national prevalence of approximately 2.6%. Sex work, sex between men and injecting drug use are the principal modes of transmission. In Ecuador, around 23,000 people are living with HIV, numbers driven in part by high migration from neighbouring Colombia's civil and drug wars, as migrants turn to sex work in an attempt to finance their resettlement. India is at a crossroads in its efforts to control HIV, which, although relatively low at 0.9%, is spreading rapidly as the country experiences multiple local epidemics. Andhra Pradesh, one of India's highest prevalence states, is also Rapalhi's home.

Rapalhi's story catalogues the catastrophic events in his life and his justifiable anger is evident, while Rosa Paladines briefly alludes to an unspoken pain, referring to her 'excellent parents' and 'beautiful babies', her wish to help others 'so they don't suffer like I did'. Then take 'Eddie'. Eddie sounds as positive as his smiling portrait suggests. He talks about his qualifications, his strength, hopes for the future, and evident pride: 'I'm a hard working man and I have always contributed to the economy of my home.' The scale of his struggle, however, becomes apparent in his photograph: a man lying on the ground, with Eddie's adjoining caption, 'Drugs are my midnight corner'. Elsewhere, Anil's picture of Ramesh stares out with beautiful, haunted eyes as he describes a solitary life spent sleeping under a tree, of being 'of little use now', Keerti has taken a picture of little Zareena wrapped in burial sheets, a young girl whose short life, filled with rape, prostitution and abuse, ended when she burned herself to death.

These stories and photographs represent the most marginalised people in their respective societies - men who have sex with men, transvestites, gay men, sex workers and others living with HIV: individuals who are the subject of strong disapproval, of social, and in some cases, legal, taboos. The exhibition has grown out of a project undertaken by the Brighton-based International Aids/HIV Alliance, which involved 45 people living with HIV in the three countries. The participants received cameras and training and used them to show us their view of the world; the idea behind the scheme was to allow those directly affected by the HIV pandemic to take control of how they are perceived by the rest of the world.

'The project is really exciting as it has the potential to dramatically increase the visibility of groups who are key to the HIV epidemic across the developing world,' says Joseph O'Reilly, project manager at the Alliance. 'Their testimonies and the images of their experiences can teach us all a lot about the ongoing stigma and discrimination that surrounds HIV and Aids, both near and far.'

The majority of the photographs and accompanying stories deal directly with the discrimination that is still, unfortunately, so widespread in many countries, such as Ashok's picture of Seema, tended to by friends after he has been beaten by 'rowdies' (local street ruffians). Or Rapalhi's heartbreakingly simple portrait of Dasu, 'kept separate from his family because of his health'. Some articulate hopes for a future free of discrimination, where 'sray sros' - a phrase commonly used by transgender Cambodians, which translates as 'charming girl' - couples could be accepted into society, and where such love could be celebrated. The participants are evidently passionate about educating their communities and changing the culture that attaches such stigma to the HIV virus, enabling it to spread devastatingly rapidly, in part because individuals affected by HIV are reluctant to claim their rights.

O'Reilly says the photographers found the experience incredibly empowering, and this was one of the project's aims - to enable those people who are traditionally the subjects of photography to become its creator. Through photography, the theory goes, individuals find confidence in their voices and are able to speak out about their challenges, concerns, hopes and fears. And the resulting exhibition would appear to back this theory up - as voice after voice talks of future plans - pursuing a photography career; running a make-up shop, working with those less fortunate.

But while the project has undoubtedly been an opportunity for some, perhaps what's more striking about the images and stories within it is the strength and positivity that evidently already existed in the most desperate of circumstances. There are characters here, such as Jahaira, who, one suspects would refuse to be cowed by any misfortune: 'I'm an adventurous and creative spirit. I'm very dynamic and have great aspirations of succeeding and being somebody in life.'

All too often Aids and HIV statistics in developing countries are just that: statistics. The people who make up those numbers, who are directly affected by the virus, and who should be at the forefront of the fight against it, are missing from public discussions about the HIV pandemic. Those people are forgotten by their own governments, health and other service providers in the response to the epidemic, and are all but invisible to the Western world. Unheard Voices, Hidden Lives is one step towards changing that.

· Unheard Voices, Hidden Lives is at the Jubilee Library, Brighton from November 13 to December 4; a book containing all of the participants' photographs and testimonies can be downloaded here. Watch a slideshow of the exhibition and the participants' stories here